Ubiquitin is a tag that often initiates
degradation of proteins by
the proteasome in the ubiquitin proteasome system. Targeted expression
of K6W mutant ubiquitin (K6W-Ub) in the lens results in defects in
lens development and cataract formation, suggesting critical functions
for ubiquitin in lens. To study the developmental processes that require
intact ubiquitin, we executed the most extensive characterization
of the lens proteome to date. We quantified lens protein expression
changes in multiple replicate pools of P1 wild-type and K6W-Ub-expressing
mouse lenses. Lens proteins were digested with trypsin, peptides were
separated using strong cation exchange and reversed-phase liquid chromatography,
and tandem mass (MS/MS) spectra were collected with a linear ion trap.
Transgenic mice that expressed low levels of K6W-Ub (low expressers)
had normal, clear lenses at birth, whereas the lenses that expressed
high levels of K6W-Ub (higher expressers) had abnormal lenses and
cataracts at birth. A total of 2052 proteins were identified, of which
996 were reliably quantified and compared between wild-type and K6W-Ub
transgenic mice. Consistent with a delayed developmental program,
fiber-cell-specific proteins, such as γ-crystallins (γA,
γB, γC, and γE), were down-regulated in K6W-Ub higher
expressers. Up-regulated proteins were involved in energy metabolism,
signal transduction, and proteolysis. The K6W-Ub low expressers exhibited
delayed onset and milder cataract consistent with smaller changes
in protein expression. Because lens protein expression changes occurred
prior to lens morphological abnormalities and cataract formation in
K6W-Ub low expressers, it appears that expression of K6W-Ub sets in
motion a process of altered protein expression that results in developmental
defects and cataract.